The technology of imaging fingerprints by measuring the differences between the coupling capacitance formed between ridgelines of fingerprints and planar sensing electrode array units and that formed between valley lines of the fingerprints and the planar sensing electrode array units is first seen in the patent application U.S. Pat. No. 4,353,056A (Siemens, 1980). Over the past 30 years, sensor technologies for imaging fingerprints based on the measurement of coupling capacitances are continuously developed. Famous enterprises involving this filed include Siemens, AT & T Bell, Philips, Toshiba, ST, NEC, Motorola, Sharp, Intel, Epson and countless venture capital companies.
Most of the capacitive fingerprint sensor technologies are developed based on macro-capacitive sensor circuit prototypes. However, the technical laws of sensors determine that capacitive sensors area combination of circuits and sensor equations, and the circuit scales and the implementation processes of circuits determine the value ranges and the tolerance ranges of various parameters in the sensor equations. When the technical solutions originally formed by macro-scale electronic components are formed by micro-scale electronic components instead, most of such technical solutions will deteriorate in sensitivity and performance in noise characteristics.
Further, capacitive fingerprint sensors as array sensors are also sensitive to the mismatching among the units, which is more difficult to control for micro-scale electronic components with respect to macro-scale electronic components. Meanwhile, since a measurement circuit is often large in size, array sensors are generally designed for single-channel, and the respective units reuse the measurement circuit in a time division manner. In order to ensure a certain image frame rate, prolonging the sampling time, which is an important method for improving signal gain in the design of array sensors, is greatly limited, or even becomes unfeasible for a single-channel array sensor with a relatively large number of points.
Such technical restrictions determine that only a handful of technical routes have the potential to move towards commercialization, and such technical routes that do not meet objective technical laws will move towards extinction. By 2013, there are three types of capacitive fingerprint sensors with certain scale of application: a radio frequency response type, which measures the amplitude of a reflected RF signal, may be represented by a US company Authentec (U.S. Pat. No. 5,828,773A) acquired by Apple Inc. in 2012, and is mainly used iniPhone5S of Apple Inc.; a transient response type, which measures a transient coupling level, may be represented by a Swedish company Fingerprint Cards (US20080069413A1), and is mainly used in the teller systems of China's state-owned banks; and a charge transfer type, which may be represented by a China's Taiwanese company Egistec (U.S. Pat. No. 7,099,497B2), and is mainly used in ideaPad of Lenovo company. The former two types are called as active types in the industry, and the third type a passive type. The common feature of the three types is that a capacitance is converted into a voltage for measurements. From the view of classification of sensors, these three types are classified as “C-V” type sensors.
In recent years, in CMOS photosensors with millions of pixels, to meet the requirements for growing image point numbers, such photosensors develop from single-channel photosensors to multi-channel ones. The so-called multi-channel array sensor, in fact, is a plurality of independent single-channel array sensors combined spatially. Taking into account the limitations of the actual circuit layout, as the number of measurement circuits changes from one into a plurality, the sizes of the measurement circuits must be greatly reduced. A “V-T” ADC is a new analog-digital converter. Compared with a direct type ADC, a “V-T” ADC has the advantage of a greatly reduced circuit scale under the same resolution requirement, and has the disadvantage of longer sampling and holding time. Multi-channel CMOS sensors tend to adopt “V-T” converters, and balance among the number of channels, the sampling and holding time and the circuit size so as to determine the best design solution.
The invention patent application filed by Chengdu Microarray Electronic Co., Ltd. in 2012 with the title of “capacitive distance sensor” and the application number of CN201210403271.2 discloses a novel” C-V-T “type capacitive distance sensor. The method adopted by the above sensor is as below: fingerprints are placed on the surface of an object to be measured (equivalent to the target electrode); a measuring capacitance (equivalent to the target capacitance) is formed by coupling between the capacitance measuring plate (equivalent to the sensing electrode) and the surface of an object to be measured; the distances from the surface of the object to be measured (equivalent to the target electrode) to the capacitance measuring plates (equivalent to the sensing electrode) of different array units are different, and so do the measuring capacitances (equivalent to the target capacitance) formed by coupling; a capacitive coupling plate (equivalent to the driving electrode) is driven by a first programmable level generator (equivalent to the level driver) so that the potentials of the capacitance measuring plates (equivalent to the sensing electrode) increase, and the increase degrees of the potentials are different as the measuring capacitance (equivalent to the target capacitance) connected in parallel are different; a reference capacitor (equivalent to the integrating capacitor) is first fully charged, and then discharges to the capacitance measuring plates (equivalent to the sensing electrode) with increased levels; and the discharging degrees are different as the potentials of the capacitance measuring plates (equivalent to the sensing electrode) are different; discharging is repeated so that the potential of the reference capacitor (equivalent to the integrating capacitor) is continuously decreased; as the decrease rates are different, the time for generating a threshold level by a second level generator (equivalent to the reference level) is different, so that the comparator outputs flip at different timings; the number of discharging times corresponding to the time the comparator outputs the flip is an output of the capacitive distance sensor.
The measuring function of the sensor is deemed as “S-C-V-T”. That is, a distance from the fingerprints to a sensing electrode is an independent variable, and the counted value in the time direction is the value of the function. The measuring function has the advantages of anti-shifting and linearity. By taking advantage of improved resolution and reduced thermal noise, the sensor has greatly improved performance. Tape-out verification results show that the technical level of the sensor is higher than the internationally advanced level in 2012 under the same process conditions, but lower than the internationally advanced level in 2013 or the technical level of Touch ID sensors installed in iPhone5S.
Due to active research, development and application of fingerprint sensor technologies by Apple Inc., the consumer electronics market has huge demand and higher requirements for fingerprint sensor technologies. The present application has improved the technical solution disclosed by the patent application CN201210403271.2 by establishing and analyzing equations for the sensitivity of sensors. The present application has introduced more items and corresponding circuits for increasing sensitivity, while proposing general models for some circuits and simplifying some circuits.